1. Field of the Invention
The present invention relates to a damper which is interposed between structures adapted to undergo relative movement and which functions as a stopper for preventing a sudden displacement of the structure. More particularly, the present invention concerns a so-called viscous shear-type damper which makes use of viscous shear occurring between fixed and movable resistance plates which are arranged in parallel with very small gaps therebetween, and are adapted to undergo relative movement via a viscous material filled in the gaps.
2. Description of the Related Art
In Japanese Utility Model Application Publication No. 43262/1979 (hereafter referred to as the prior art), the present assignees earlier proposed a viscous shear-type damper of this type as an "apparatus for causing damping action."
Namely, the apparatus for causing damping action according to the aforementioned prior art is characterized by the following. Fixed resistance plates formed of rectangular thin plates and moving resistance plates or movable resistance plates formed of thin plates whose length is shorter than that of the fixed resistance plates and whose transverse width is wider than that of the fixed resistance plates are arranged alternately with very small gaps therebetween. The moving resistance plates have mutually opposing cutout portions at their transverse end portions, and engaging blocks are engaged in the cutout portions. A viscous material is filled between the resistance plates thus assembled, and the entire assembly is accommodated in a casing. The fixed resistance plates at their longitudinal end portions abut against casing walls, whereas the moving resistance plates engaged by the engaging blocks have spaces with respect to the casing walls in their transverse end portions. The engaging blocks and the casing are respectively fixed to mutually opposing structural members.
As such, the above-described apparatus in accordance with the prior art has the following features:
(1) Fluid pressure does not take place in the apparatus, and leakage of a fluid does not occur, so that it is unnecessary to adopt a hermetically sealed structure for the apparatus.
(2) Since the apparatus does not have fitting and sliding portions similar to those of a piston cylinder-type damper, a high level of processing accuracy is not required.
(3) Even if the engaging blocks have slight inclinations, parallel positional relationships are maintained between the fixed resistance plates and the movable resistance plates, and a factor of resilient instability of the moving resistance plates in the principal moving direction, in particular, is decreased appreciably.
However, in the above-described prior art, bubbling due to entrainment of air occurs in the viscous material during the operation, i.e., during a sudden displacement of the moving resistance plates. At times, there are cases where overflow can occur, making it impossible to allow stable viscous-shear resisting force to be demonstrated sufficiently.